To provide a light weight polyester fiber excellent in both appearance and physical characteristics (strength, elongation, etc.), requiring no specific spinneret and spinneret pack and generating no fiber deformation and fiber wall rupture even during the spinning process and after processing.

This light-weight polyester fiber comprises a polyester resin and a styrene-maleimide resin having 60000-170000 molecular weight. The blending amounts of the polyester resin and the styrene.maleimide resin are 97-70 wt.% and 3-30 wt.% respectively. The polyester fiber has minute voids and the apparent specific gravity of the fiber is ≤1.0.